The present invention relates to improvements in and relating to a process for the manufacture of microcapsules of specifically selected agricultural chemicals as of organo-phosphoric acid derivatives adapted for use as agricultural pesticides.
Specifically selected organo-phosphoric acid derivatives such as dimethyl-2,2-dichlorovinyl phosphate (DDVP), dimethyl-1,2-dibromo-2,2-dichloroethyl phosphate are liably subject to hydrolysis and thus rapidly decomposed in natural environment. These compounds are used, therefore, broadly as agricultural pesticides showing least residual phytotoxicity. On the other hand, these compounds represent only short effective term upon application, on account of their high liability to hydrolysis, and thus, they must be applied frequently and repeatedly in practice. In order to avoid this defect, it has been also tried to capsulate these chemicals for the purpose of supressing the hydrolytic liability and to prolong the effective duration term.
For the microcapsulation of organo-phosphoric acid derivatives as the capsule core, the conventional complex coacervation process has hitherto been relied upon. It has been experienced, however, that with such a process as above, the desired microcapsules can not be obtained with high yield.
Taking DDVP as a representative example, the compound shows only about 1% of solubility to water under normal temperature and when it is dissolved in water, the compound is liably subject to hydrolysis, thereby providing phosphoric acid. Therefore, in this case, the solving process in water will continuously progress. Even if the quantitative ratio of DDVP to the aqueous phase is set to 1:10 or so, the capsulation efficiency will be less than 90%.
On the other hand, DDVP dissolves about 10% at normal temperature, the hydrolysis will progress within the capsule upon the formation thereof and the capsulation yield will be about 50% or so. In addition, the preservation of the capsules is highly inferior.
Further, since DDVP is subject to hydrolysis, phosphoric acid is formed and the pH of the aqueous phase will be lowered liably to 3.0 or so by the presence of the formed phosphoric acid, thereby the particle size of the produced microcapsules becoming uneven by virtue of the thus invited unstable and unhomogenized condition of the aqueous emulsion phase. It has been experienced that in prosecution of the complex coacervation process, the range of pH value adapted for the accumulating formation of capsule shell membrane is preferably between 3.9-4.3. Therefore, if the pH value of the aqueous dispersion phase should drop below the above preferable range, adjustment is necessary by addition of alkaline solution. By execution of such alkali addition procedure for pH-concentration adjustment, a neutral salt will develop and adversely affect upon the yield of shell membrane formation by the accumulation of polycationic colloid.
Several prior proposals for providing counter measures against above kind drawbacks have been already published, for instance, in Japanese Patent Publication Nos. Sho-37-12381 and Sho-37-15761, for providing multi-layer microcapsules. According to these proposals, such a measure is adopted during microcapsulation of a hydrophilic substance in the complex coacervation process, a preparatory protecting coating composed of hydrophobic high molecular ethyl cellulose, so as to provide finally a multi-layer microcapsules. However, it has been experienced that when employing such prior proposals for the microcapsulation of organo-phosphoric acid derivatives such as DDVP, many difficulties are met and the desired effect can not practically be attained.